Device including magnet-biased magnetoresistive sensor and rotatable, magnetized encoder for detecting rotary movements

ABSTRACT

A device for detecting rotary movements includes a measuring data emitter that co-rotates with the part performing the rotary movement, and a stationary transducer which includes a magneto-resistive sensor element with a bias magnet. The measuring data emitter is an encoder having successive permanent magnet areas of alternating polarity in the direction of rotation (Y direction). The sensor element is arranged and the bias magnet is magnetized in such a way that, during rotary movement, in a preferred direction of the sensor element (Y direction) which is orthogonal to the magnetic field component (X direction) produced by the bias magnet, the magnetized areas cause a variable magnetic field component between the adjacent permanent magnet areas which is representative of the rotary movement and passes through the magneto-resistive sensor element in the Y direction.

TECHNICAL FIELD

The present invention relates to a device for detecting rotary orangular movements.

BACKGROUND OF THE INVENTION

Devices of this type are often required on automobiles to monitor therotational behavior of the individual vehicle wheels. Wheel speed is ofcritical importance as an input variable for control systems, such asanti-lock systems, traction slip control systems and driving stabilitycontrol systems.

Sensor devices to determine the rotational speed of wheels are known ina great number of applications. Normally, the measuring data emitter insuch sensor devices is an incremental encoder which is coupledmechanically to the component or wheel the rotation of which is to bemeasured. Further, there is a transducer or sensor which scans theencoder. Ferromagnetic toothed wheels, toothed rings, ferromagnetic holediscs, etc., are used as encoders. When sensors are used in wheelbearings, it is also customary to employ magnetized structures as ameasuring data emitter, for example, annular or circular arrangements ofsuccessive north/south poles, embedded in a mechanic carrier member.

It is also common to use so-called "passive" sensors according to thereluctance principle. These sensors use a copper coil with a permanentmagnet as a transducer. The transducer is coupled magnetically to thetoothed disc serving as measuring data emitter, or to any other encoder.The encoder modulates the magnetic coupling reluctance synchronouslywith movement, and an alternating voltage representative of the movementis induced in the copper coil. The frequency of the alternating voltagepermits being evaluated as a measured quantity to determine therotational wheel speed. The magnitude of the induced signal voltage is afunction of the rotational speed and the air slot between the measuringdata emitter and the transducer or between the tooth system and thesensor.

"Active" sensors, which also cover the subject matter of the presentinvention, are well known in the art. In principle, they are combined ofa magneto-statically sensitive element and a permanent magnet which ismagnetically coupled to the encoder. The encoder modulates,synchronously to movement, the magnetic coupling reluctance or the fielddirection, and the sensor element responds to the variation of the fluxdensity or the movement of a field vector. Examples in the art of suchmagneto-statically responsive elements are Hall probes andmagneto-resistive structures on the basis of permalloys. The magnitudeof the signal voltage on the sensor element is responsive to the airslot, but independent of the rotational speed or the frequency.

The publication entitled "Magneto-resistive rotational speedsensor--reliable and inexpensive" (Graeger, Petersen, Elektronik24/1992, pages 48 to 52) describes an active rotational speed sensor ofsuch a type which interacts with a toothed disc made of ferromagneticmaterial that is the measuring data emitter. The actual sensor elementincludes a permanent magnet on the side remote from the toothed disc.The magnetic field of this magnet serves to bias the magneto-resistivesensor element and to produce the magnetic coupling reluctance with theco-rotating toothed wheel. Therefore, a permanent magnet with arelatively large volume is required to permit an air slot that issufficient in practical operations.

An object of the present invention is to provide a device of thepreviously mentioned type including an active sensor element which,compared to known devices of this type, necessitates a minimum possiblestructural volume to permit accommodation of the device in a wheelbearing of an automotive vehicle, for example, and which also permits alargest possible air slot or distance between the measuring data emitterand the transducer.

A special feature of the device of the present invention is that themeasuring data emitter has successive permanent magnet areas ofalternating polarity in the direction of rotation, and that the sensorelement is arranged and the bias magnet is magnetized such that the biasmagnet produces a field component which extends vertical to thedirection of movement of the measuring data emitter in a preferreddirection of the sensor element, and that during a rotary movement, in apreferred direction of the sensor element which is orthogonal to themagnetic field component produced by the bias magnet, the magnet areascause a varying course of field strength between the adjacent permanentmagnet areas which passes through the sensor element in the direction ofrotation and represents the rotary movement.

A magneto-resistive sensor element is used in the present inventionbecause it is known to permit, under comparable conditions, larger airslots between the encoder and the sensor compared to Hall elements. Thesensor element KMI 10/1 cited in the above-mentioned publication(Elektronik 24/1992) is an example of a like active rotational speedsensor element. The above-mentioned sensor element is intended for usein combination with an encoder of any ferromagnetic material. As isshown in the attached FIG. 1, the sensor comprises a magneto-resistiveresistance bridge 1, an electronic evaluating circuit 2 and a permanentmagnet 3 which is magnetized in the direction of the XZ plane. Inoperation, the sensor is coupled magnetically over a small air slot to atoothed wheel made of ferromagnetic material. The teeth are oriented inthe X direction (see FIG. 1) and move on rotation of the toothed wheelin the Y direction past the side of the bridge, comprising themagneto-resistive resistors, that is opposite to the permanent magnet 3.The result is an alternating deformation of the magnetic field of the XZplane additionally in the Y direction. The magneto-resistive bridge isso designed and arranged that it reacts to the field strength componentin the Y direction by mistuning. The electronic evaluating circuit 2mainly comprises a bridge signal amplifier with a subsequent triggercircuit which produces a binary output signal with two constantamplitude values in the area of the nominal air slot irrespective of thesize of the air slot. The change in flanks of the amplitude valuesrepresents the division of the toothed wheel of the measuring dataemitter. The evaluating circuit is designed so as to issue the signal inthe form of a current through terminals 4. Both the sensor element andthe signal-processing circuit are integrated circuits that areencapsulated in a plastic housing each. The two housings aremechanically interconnected by a system carrier member 10. Also, thesystem carrier member provides the electrically conductive connections.

The device of the present invention is based on the teaching that thedesired reduced structural, volume and, additionally, a largeradmissible air slot can be achieved by subdividing the magneticstructure into a bias magnet, which produces a magnetic field in the Xdirection, that is, in one of the preferred magnetic directions of themagneto-resistive sensor element, which extends orthogonally to thedirection of movement of the encoder (Y direction), and the permanentmagnet areas of a measuring data emitter.

In a preferred embodiment of the device of the present invention, themeasuring data emitter has the shape of a disc performing the rotarymovement. More preferably, the disc-shaped emitter is cylindrical inshape having an inner and outer periphery. The permanent magnet areasare distributed evenly over the periphery of the disc. Preferably, thepermanent magnet areas are embedded on the outer edge periphery of thedisc. Alternatively, the permanent magnet areas are embedded on theinner periphery of the disc. Appropriately, the permanent magnet areasare embedded in a mechanic carrier material or they are produced bymagnetization of areas. The transducer is aligned in parallel to thedisc or paraxially depending on the arrangement of the areas.

Further favorable embodiments of the present invention are set forthherein. For example, the transducer may be configured as a one-housingor a two-housing basic element, and the bias magnet may be arranged onthe housing, especially on the side remote from the encoder, or it maybe embedded into the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view and a side view of the basic design of thepreviously described prior art transducer including an active sensorelement.

FIG. 2 is a perspective, schematically simplified view of the outsidedesign of an embodiment of a transducer of the device of the presentinvention.

FIG. 3 is a view, similar to FIG. 2, of a second embodiment of thedevice of the present invention in combination with an encoder.

FIG. 4 is a cross-sectional schematic view of the basic design of anembodiment of the transducer of FIG. 2.

FIG. 5 is a view, similar to FIG. 4, of an alternative embodiment of thetransducer of the present invention.

FIG. 6(a) and FIG. 6(b) are schematic, partial cross-sectional views ofthe arrangement of a transducer relative to a measuring data emitter ina prior art measuring device (FIG. 6(a)) and a measuring deviceaccording to the present invention (FIG. 6(b)).

FIGS. 7 and 8 are perspective views of various arrangements of the dataemitter and the transducer.

FIG. 9 is a cross-sectional view of the data emitter and transducer ofthe present invention assembled in a wheel bearing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In contrast to FIG. 1, FIG. 2 shows a transducer 14 for a device of thetype of the present invention. The embodiment of FIG. 2 relates to atwo-housing transducer accommodating in a sensor housing 5' amagneto-resistive sensor element and a bias magnet which has a verysmall size compared to the state of the art, and in an IC housing 6 anevaluating circuit which is preferably provided by an integratedcircuit. The entire assembly unit is preferably mounted on a commonsystem carrier member 10, similar to the system carrier member ofFIG. 1. The reference directions X, Y, Z which are used to explain thepresent invention more closely are identified in FIG. 2. `Y` refers tothe direction of movement or the direction of rotation of the associatedencoder (see FIG. 3), `X` refers to the preferred magnetic direction ofthe sensor element 1 which is orthogonal to `Y` and defines thepolarizing direction of the bias magnet (7, 11; see FIGS. 3 to 5).Finally, `Z` refers to the third reference direction of the coordinatesystem used in this respect.

FIG. 3 illustrates the relative arrangement of the transducer 15 and theassociated measuring data emitter 8 of the present invention. Incontrast to the embodiment of FIG. 2, the bias magnet 7 in FIG. 3, whichis polarized in the X direction, is arranged outside the sensor housing5. Sensor housing 5 accommodates the sensor element 1 (not shown).

As is shown in FIG. 3, a magnetic encoder 8 is used as a measuring dataemitter in the present invention. Encoder 8 is a disc having a pluralityof permanent magnet areas 9 spaced evenly and distributed evenly on theinner periphery of the disc on the rotational surface. The areas 9,which are magnetized or embedded in a carrier material 17, formalternating successive north and south poles N/S in the direction ofmovement or Y direction. When the device of the present invention isconfigured as a bearing sensor, suitably, the encoder is a wheel bearingsealing ring magnetized as shown in FIG. 3.

When the magnetic encoder 8 of the embodiment of FIG. 3 is moved in theY direction, an electric signal voltage develops at terminals A/B whosefrequency is determined by the sequence of the north pole south polealternation. Because the sensor in this arrangement is an active sensor,an electric energy source is required, which is identified by the sourceQ.

FIG. 4 is a schematic view of the internal construction of thetransducer, which is shown in a perspective view in FIG. 2. Atwo-housing basic element is seated on the carrier structure member 10.The housing part 5' accommodates the magneto-resistive element 1 and thebias magnet 11. The bias magnet 11 is magnetized in the X direction andis arranged on the side remote from the magnetized encoder 8 (see FIG.3). Thus, the sensor element 1 is arranged between the bias magnet 11and the magnet areas 9 of the encoder or measuring data emitter 8. Theevaluating circuit 2 is accommodated in the second housing part 6.

The transducer of FIG. 5 differs from the embodiment of FIG. 4 by thepositioning of the evaluating circuit 2'. The electronic circuit 2 inthis embodiment is incorporated between the sensor element 1' and thebias magnet 11. The carrier structure member 10', in which theelectrical connections are included, is interposed between the circuit2' and the bias magnet 11. Other arrangements are also possible. Aboveall, it is important that the permanent magnetic field of the biasmagnet 11 extends through the sensor element 1' in the X direction andthat the sensor element 1' is exposed to the magnetic field of themagnet areas 9 of the encoder 8 in the Y direction (not shown).

The outside configuration of the bias magnet 11 is not limited to adefined shape. What matters is the premagnetization in the X direction.The type of shape may then be chosen by taking into consideration themethod of manufacture or the installation of the transducer. A smallvolume is sufficient for the bias magnet 11 because its field is onlyrequired to pass through the sensor element 1, 1', but does not have tobridge the air slot to the measuring data emitter 8.

A major advantage of the measuring device of the present inventioncompared to prior art devices having an active sensor element and aferromagnetic measuring data emitter is the reduced structural volumenecessary. Only this reduction in volume permits the design of a wheelbearing sensor according to this principle in practical operations.

FIG. 6 shows the differences between the state of the art and thepresent invention. In the known principle in FIG. 6a, a large-volumepermanent magnet 3 is required whose magnetic field in the Z directionmust reach beyond an air slot L₁, up to the ferromagnetic measuring dataemitter 18. In FIG. 6b, showing the arrangement of the presentinvention, a bias magnet 11 of a much smaller size is sufficient. It hasshown in practical operations that a permanent magnet diminished in sizeby the factor 100 is sufficient for the device of the present invention.

The admissible air slot between the sensor element 1 and the surface ofthe magnetized encoder 8 may become considerably larger in the device ofthe present invention than that of the prior art rotational speedmeasuring device of FIG. 6a.

Thus, the permanent magnet 3 of the prior art device has to produce amagnetic field over the distance L₁, with only the distance L₂ beingavailable as an air slot (when neglecting the housing in which thesensor element 1 is accommodated). In contrast thereto, L₁ =L₂ appliesin the device of the present invention.

FIGS. 7 and 8 show various arrangements of the permanent magnet areas 9on the encoder 8 with respect to the transducer 15 of the presentinvention. In one embodiment as shown in FIG. 7, the permanent magnetareas 9 are arranged on the outer edge periphery of the encoder 8. Inthis configuration, transducer 15 is arranged parallel to the rotationalplane of encoder 8 and perpendicular to the rotational axis.

In an alternative embodiment as shown in FIG. 8, the permanent magnetareas 9 are arranged on the inner periphery of encoder 8. In thisconfiguration, transducer 15 is arranged paraxially to the rotationalaxis of the encoder 8.

FIG. 9 shows an example of the data emitter/transducer assemblyinstalled in a wheel bearing according to the present invention. Asshown in FIG. 9, data emitter 8 of the present invention is installedinto the wheel bearing. The transducer 15 is then installed in closeproximity to magnet areas 9 of the data emitter 8 such that the magneticflux is detected when the wheel rotates.

The arrangements as shown in FIGS. 7, 8, and 9 are for illustrativepurposes only and other arrangements of the encoder 8 and transducer 15in a wheel bearing may be employed without departing from the scope ofthe present invention. Accordingly, the preceding description is notintended to be exhaustive or to limit the invention to any precise formdisclosed. Many modifications and variations are possible in light ofthe above teaching.

The preferred embodiment was chosen and described in order to bestexplain the principles of the invention and its practical application.The preceding description is intended to enable others skilled in theart to best utilize the invention in various embodiments and withvarious modifications as are suited to the particular use contemplated.It is intended that the scope of the invention be defined by thefollowing claims.

I claim:
 1. A device for detecting rotary movements, comprising:adisc-shaped data emitter mounted in a wheel bearing including successivepermanent magnet areas distributed evenly over a periphery of the dischaving polarities alternating in the direction of said rotary movementand oriented in a direction perpendicular to the direction of saidrotary movement, and a stationary transducer having a first side facingsaid data emitter and a second side facing perpendicularly away fromsaid data emitter and spaced from said data emitter, said stationarytransducer including a magneto-resistive sensing element and a biasmagnet for providing a bias magnetic field to the sensing element,wherein said bias magnet is polarized in a direction perpendicular tothe direction of said rotary movement and to the direction oforientation of the polarities of the permanent magnet areas, whereinsaid bias magnet is mounted on said second side of said stationarytransducer to minimize the thickness of said stationary transducer. 2.Device as claimed in claim 1, wherein the permanent magnet areas arearranged in the rotational plane perpendicular to the rotational axis ofthe disc on an outer edge periphery of said disc, and the stationarytransducer is aligned in parallel to the rotational plane of the disc.3. Device as claimed in claim 1, wherein said disc is cylindrical inshape having an inner and outer periphery, the permanent magnet areasare arranged over the inner periphery of the disc, and the transducer isaligned paraxially with the disc.
 4. Device as claimed in claim 1,wherein the permanent magnet areas are provided by embedding magneticmembers in a mechanic carrier material or by magnetizing areas of acarrier.
 5. Device as claimed in claim 1, wherein the transducer iscomprised of a two-housing basic element attached to a mounting carrier,and wherein the bias magnet is arranged outside the housing of themagneto-resistive sensor element.
 6. Device as claimed in claim 5,wherein the transducer is configured as a one-housing basic element, andin that the bias magnet is arranged outside the housing on the side ofthe housing remote from the measuring data emitter.
 7. Device as claimedin claim 5, wherein the transducer is configured as a two-housing basicelement, and in that the bias magnet is arranged inside the housing ofthe magneto-resistive sensor element.
 8. Device as claimed in claim 5,wherein the transducer is configured as a one-housing basic element, andin that the bias magnet is arranged inside the basic element.
 9. Deviceas claimed in claim 5, wherein the bias magnet is arranged inside oroutside the housing relative to the magneto-resistive sensor element onthe side remote from the measuring data emitter.
 10. Device as claimedin 1, wherein said data emitter is provided by incorporating permanentmagnet areas in a wheel bearing sealing ring or by magnetization ofcorresponding areas.